1. Field of the Invention
This invention relates to the field of wireless communication systems. More specifically, the invention relates to power management systems for mobile units.
2. Description of the Related Art
The use of wireless communication systems is growing with users now numbering well into the millions. However, one inconvenience associated with routine use of a mobile unit is the constant need to recharge and replace depleted batteries. Even users who make and receive a few telephone calls such that their mobile units operate mostly in a standby mode (awaiting calls) experience the annoying and frequent problem of depleted battery power.
As a mobile unit travels through a cellular network, the mobile unit moves through service areas known as cells. Each cell is a specific geographic region containing a base station. When moving from one cell to another, the base station servicing the mobile unit changes from the base station of one cell to the base station of another cell. In many analog cellular systems, this change is controlled by the base stations and the switch is called xe2x80x9chandoff.xe2x80x9d However, in conventional digital cellular systems, the mobile unit assists in determining when the serving base station should be changed, and the switch is termed xe2x80x9chandover.xe2x80x9d
In digital cellular systems, the mobile unit does not have to be served by the nearest base station. From signal strength measurements, the mobile unit can determine which base stations are providing signals capable of adequately servicing the mobile unit. This information is then sent to a mobile switching center to determine which base station will serve the mobile unit. Due to loading requirements, it may be advantageous for a more remote base station to serve the mobile unit, provided the received signal strength from the remote base station is adequate.
A large portion of battery power consumed in common standby modes is attributable to determining proper handover. While in the standby mode, the mobile unit is periodically activated to scan the signal strength of each neighboring cell. For example, in the Global System for Mobile Communications (GSM) wireless communication networks, a mobile unit receives and decodes the signal strength of each neighboring cell about once every thirty seconds.
At any given time, a mobile unit may have between 6 to 12 neighboring cells. Because of the requirement to scan each cell approximately every 30 seconds, the mobile unit may be activated every 2-5 seconds. Each activation and scan consumes a significant amount of battery power, thereby reducing the standby time of the mobile unit.
Improvements in battery technology, while helpful, have done little to avoid the seemingly ever-present need to recharge and replace mobile unit batteries. What is needed is a system to conserve battery power by minimizing the power consumed scanning neighboring cells.
The present invention reduces power consumed by a mobile unit in the standby mode by reducing the frequency the mobile unit scans neighboring cells to determine signal strength. Reduced scanning of neighboring cells consumes less power and advantageously increases the standby mode lifetime of a mobile unit battery.
For example, scanning each neighboring cell every 30 seconds ensures a mobile unit traveling through a cell knows which base stations are providing usable signals. In this manner, when the mobile unit crosses a cell boundary, or loses the required signal strength from its servicing base station, the mobile switching center can handover the mobile unit to a new base station.
Generally, a handover occurs when a mobile unit exceeds the range of its servicing base station. Therefore, if a mobile unit is stationary, or moving at a slow rate of speed, there is less need to monitor the neighboring base stations.
One embodiment of the invention detects the speed the mobile unit is moving and adjusts the frequency of scanning each neighboring cell accordingly. For example, if a mobile unit is moving slowly through a cell, the time period between scanning for each neighboring cell can be increased.
The speed of the movement through a cell may be determined by the change in signal strength. Every time a mobile unit communicates with the base station, the received signal strength is measured. If a mobile unit is not moving through the cell, the signal strength should remain fairly constant. However, as the mobile unit moves away from the base station, the signal strength decreases.
One embodiment of the invention determines the rate of change in the measured signal strength of the mobile unit. If the rate of change is low, the mobile unit increases the amount of time between each scan of a neighboring cell.
One embodiment of the invention is a wireless communication system comprising a plurality of base stations which transmit signals and a mobile unit which intermittently detects the signals transmitted by the plurality of base stations. A signal strength detector then determines the quality of the signals received by the mobile unit, and a processor calculates the speed at which the mobile unit is moving from one of the plurality of base stations based on the rate of change of the quality of the signals received by the mobile unit. The processor adjusts the frequency in which the mobile unit detects the signals transmitted by at least one of the plurality of base stations based upon the speed of the mobile unit.
One embodiment of the invention is a method of conserving power in a wireless communication system. The method comprises the acts of measuring the quality of a plurality of signals received from one of a plurality of base stations and then calculating the speed of a mobile unit from the signal quality measurements. The mobile unit then adjusts the frequency the signals are detected based upon the speed of the mobile unit.
In one embodiment of the invention, a wireless communication system comprises a speed sensor which determines the speed of a mobile unit based upon the rate of change of the strength of signals received by a mobile unit. A scan inhibitor causes the mobile unit to inhibit detection of at least one message from a base station when the speed of the mobile unit is below a set level.
One embodiment of the invention is a method of saving power in a communications system. The method comprises the acts of determining the speed of a receiving unit and altering a periodic interval for detecting a transmission based upon the speed of the receiving unit.
In one embodiment of the invention, a wireless communication system comprises means for determining a change over time of a signal and means for adjusting the rate of detecting the signal based upon the change over time.
One embodiment of the invention is a method of saving power in a communications system. The method comprises the acts of measuring signal changes as a function of time and then altering a periodic interval for detecting a transmission based upon the measured signal changes.